1. Field of the Invention
As described herein, the invention relates generally to electrophotographic machines having endless photosensitive members on which latent images are formed and, more particularly, to improved arrangements for driving the endless photosensitive member.
2. Description of the Prior Art
In electrophotographic copying machines of the type using as the image-retaining element a photosensitive surface on a sheet material, an electrostatic latent image being formed thereon upon exposure to patterned light rays, it is known to mount a photosensitive material in an endless or joined form on a circumferential surface of a conductive supporting drum -- made of metal or the like -- by an adhesive or other suitable means and exposing the photosensitive material to a light image through a slit while rotating the supporting drum. On the other hand, in another known method which is not limited to the slit exposure as in the first-mentioned exposure method, the photosensitive material is provided in the form of a belt with a wide planar photosensitive surface to allow instantaneous exposure of a full frame.
In this second-mentioned exposure method, however, it is an essential requisite that the exposure area of the photosensitive surface be maintained in a plane in order to allow the desired full frame exposure and that the photosensitive material be maintained under predetermined tension conditions by means of rollers adapted to support and drive the photosensitive material while precluding slipping movement or deviation of the photosensitive material on the driving rollers thereby ensuring accurate rotational movement of the photosensitive material with the driving rollers. In addition, the photosensitive body is required to maintain its smooth plane surface in the exposure area during repeated electrostatic image forming operations. In general, the photosensitive body is required to withstand 5000 to 10,000 cycles of copying operation.
Where a hard or relatively rigid metallic material is used for the base of the photosensitive body in order to satisfy the above-mentioned requirements, relatively large forces are necessitated in extending and tensioning the photosensitive body to obtain a plane surface. The photosensitive surface of that member (for example, a vapor-deposited organic photoconductive surface layer) is then subject to cracking due to repeated bending on and along the curved surfaces of the driving and supporting rollers. This is mainly attributable to the rigidity of the base material of the photosensitive body which hardly fit the curved surfaces of the driving rollers and causes cracks to form in the photosensitive surface -- the organic photoconductive layer say -- as a result of the physical bending movement. Especially where the driving and support rollers are required to have small diameters for the sake of compactness of the duplicator machine, such cracking occurs to a greater degree which is not tolerable in practical applications.
For these reasons, it is the general practice with the belt-type photosensitive body to employ a flexible material -- such as, for example, a polyethylene terephthalate film -- as the base of the photosensitive body. The known photosensitive body having a base of a flexible material is usually held between driving belts disposed on opposite sides of the photosensitive body and all being secured to each other by means of a transverse metal fitting at a suitable position on the circumference of the belts so that the photosensitive body and the driving belts are driven at the same speed. Difficulties are encountered, however, with such known construction. Firstly, it is difficult to apply sufficient tension on the photosensitive body due to differences in elasticity between the photosensitive body and the driving belts, that is to say, it is difficult to maintain the photosensitive body in a plane state as required for full frame instantaneous exposure. The difficulty becomes more pronounced with a metal belt.
Secondly, since the wide photosensitive body is in contact with the driving rollers over its entire width, it is difficult to control uniformly the tensions in the opposite longitudinal edge portions of the photosensitive body. Further, the difference in tension between these portions becomes prohibitively large during operation over a long period of time, thus driving the photosensitive body in a biased state. As a result, the photosensitive body loses its flatness and is deviated from a correct position, i.e., from the exposure position. This tendency of the wide photosensitive body to deviate is very strong so that the belts do not function as a guide for the photosensitive body at all, this difficulty being present even in the case where the belts are formed of metal and provided with perforations to ensure secure drive by a sprocket.
Thirdly, the metal fitting which fixedly connects the photosensitive body and the belts generally projects above the photosensitive surface so that it scratches off the powdery toner cascaded on the surface of a developing drum located at a predetermined distance from the photosensitive surface in the toner developing station, causing irregularities in development. Furthermore, projection in one particular locality gives rise to overcharging which leads to contamination of various portions due to deposition of the toner in a large amount.
Where the photosensitive body is directly provided with perforations along the longitudinal edges thereof to allow drive by a sprocket, the drive system has a simplified construction but involves positional deviation of the photosensitive body, which might destroy the perforations and shorten the life of the photosensitive body per se.
On the other hand, for the purpose of overcoming the aforementioned drawbacks, there has been proposed a drive system where instead of providing the photosensitive body in the endless form mentioned hereinbefore, it is provided in the form of a number of discrete photosensitive sheets suspended between a pair of parallel drive belts. With this drive system, however, there exists a number of large gaps between the respective photosensitive sheets in series in the circumferential direction, so that toner is allowed to get inwardly of the photosensitive body through the gaps to a position between the surface of the roller and the rear surface of the photosensitive body, forming a projection at a particular locality of the photosensitive body toward the photosensitive surface thereof since the photosensitive body is held in a tensioned state, and causing thereat an overcharging which would contaminate and make unclear the duplicated image.
The present invention is therefore directed to an apparatus which overcomes the aforementioned difficulties and defects in driving a wide endless photosensitive member having a base of a flexible material.